One type of toilet includes a pressured water source, where the pressure of water at the flush valve seat is greater than that due to a moderate height of water above the valve seat. Such toilets are often of the "low profile" type, and the toilet may be either a urinal type which has a small bowl or a sit-down type which has a toilet seat. If the inlet valve should leak, so that the water pressure in the source exceeds a predetermined level, then water must be drained from the source, either into the toilet bowl or into the toilet bowl outlet that leads to the drain. Unless the water source is designed to withstand very high city water pressures such as over 100 psi, which would make a moderate capacity water source very cumbersome and expensive, the alternative is to allow bursting of the water source and flooding of the washroom. A pressure relief valve can be placed anywhere in the water source, and its outlet can be connected to the toilet bowl or toilet bowl outlet, but such additional connections must be water tight and can add substantially to the cost of the toilet. A pressure relief valve which minimized the number of fluid tight connections that had to be made, would be of considerable value.
The force required to initially lift the flush valve member off the valve seat to begin a flushing, depends upon the pressure of water at the valve seat. A typical home gravity toilet (nonpressurized) stores water at a height of about 15 inches above the valve seat, where the water is at a pressure of about 0.54 psi. A pressure toilet may store water at a considerably higher pressure such as 1 psi. As a result, a larger force is normally required to lift the valve member off the valve seat. It is possible to use leverage so a low force can lift the valve seat, but this results in the need for the force to be applied over a longer distance. A flush valve device which could enable liftoff of the valve member using a moderate force applied over a moderate distance, would be of value for pressured water toilets.